下面一段代码，来自 http://www.scratchapixel.com/lessons/mathematics-physics-for-computer-graphics/monte-carlo-methods-mathematical-foundations

其中有 ifs >> header ,我用vs2017是编译不过的，但是相信作者肯定是编译过的，不知道作者用的什么环境，请有经验的同学指点一下

#include <fstream> 
#include <cstdlib> 
#include <cstdio> 
 
int main(int argc, char **argv) 
{ 
        std::ifstream ifs; 
        ifs.open("./tex.pbm"); 
        std::string header; 
        uint32_t w, h, l; 
        ifs >> header; 
        ifs >> w >> h >> l; 
        ifs.ignore(); 
        unsigned char *pixels = new unsigned char[w * h * 3]; 
        ifs.read((char*)pixels, w * h * 3); 
        // sample
        int nsamples = 8; 
        srand48(13); 
        float avgr = 0, avgg = 0, avgb = 0; 
        float sumr = 0, sumg = 0, sumb = 0; 
        for (int n = 0; n < nsamples; ++ n) { 
                float x = drand48() * w; 
                float y = drand48() * h; 
                int i = ((int)(y) * w + (int)(x)) * 3; 
                sumr += pixels[i]; 
                sumg += pixels[i + 1]; 
                sumb += pixels[i + 2]; 
        } 
        sumr /= nsamples; 
        sumg /= nsamples; 
        sumb /= nsamples; 
        for (int y = 0; y < h; ++y) { 
                for (int x = 0; x < w; ++x) { 
                        int i = (y * w + x) * 3; 
                        avgr += pixels[i]; 
                        avgg += pixels[i + 1]; 
                        avgb += pixels[i + 2]; 
                } 
        } 
        avgr /= w * h; 
        avgg /= w * h; 
        avgb /= w * h; 
        printf("Average %0.2f %0.2f %0.2f, Approximation %0.2f %0.2f %0.2f\n", avgr, avgg, avgb, sumr, sumg, sumb); 
        delete [] pixels; 
        return 0; 
}